1. Field of the Invention
This invention relates to a method for determining toughness against fracture (to be referred to as "fracture toughness", hereinafter) of rock by a core boring, which method is particularly useful as a means for logging of underground rock.
To exploit geothermal energy from hot dry rock, engineering to facilitate the design of underground heat-exchange surface (crack surface) is necessary. Knowledge of the fracture toughness of rock is indispensable for such engineering because it is one of the most fundamental physical properties which rule behavior of underground cracks.
2. Description of the Prior Art
ISRM (International Society for Rock Mechanics) proposes a core test method for determining fracture toughness of underground rock by using a core bored therefrom. This test method allows the use of either of two types of test piece; namely a three-point bending test piece CB with a chevron notch as shown in FIG. 1A and FIG. 1B, and a short rod test piece SR with a chevron notch as shown in FIG. 1C and FIG. 1D. Stress intensity factor K of the test piece is given as follows. For the three-point bending test piece CB: EQU K=0.25(S/T)Y.sub.c 'F/T.sup.1.5 ( 1)
For the short rod test piece SR: EQU K=fF/T.sup.1.5 . . . . . ( 2)
Here, T is the diameter of the test piece, S is the spacing between support points of the test piece, F is the load to the test piece, and Y.sub.c ' and f are correction factors.
The core test method of ISRM provides for two levels, i.e., level I and level II, from the standpoint of the ease of testing procedure.
The philosophy of the level I test for evaluating the fracture toughness assumes that a crack propagates with a constant value of the stress intensity factor K at the tip of the crack, and the fracture toughness is determined at an evaluating point where the above corrections factors Y.sub.c ' and f are minimized or a maximum load F.sub.max is applied. Crack length a.sub.c at the evaluating point depends only on the shape of the test piece. The level I test gives the following fracture toughness K.sub.CB or K.sub.SR for the above test piece. EQU K.sub.CB =A.sub.min F.sub.max /T.sup.1.5 ( 3) EQU K.sub.SR =24.0F.sub.max /T.sup.1.5 ( 4)
Here, EQU A.sub.min =0.25(S/T)[7.34+28.6(t.sub.0 /T)+39.4(t.sub.0 /T).sup.2 ]
In the level II test, non linearity correction is applied to the fracture toughness K.sub.CB and K.sub.SR obtained by the level I test. It is proposed to determine an evaluating point load F.sub.c which corresponds to a critlcal crack length a.sub.c based on an unloading compliance method. FIG. 2 shows load-displacement (F-.delta..sub.F) curves for repeated load-unload cycles. Compliance of a test piece at a load stage F.sub.H is defined as the slope of a straight line which passes through both a point H for the load stage F.sub.H and a point L for one-half of the load stage (0.5F.sub.H).
Based on such linearized compliance, the evaluating point load F.sub.c and a non-linearity correction factor p are determined, and the fracture toughness K.sup.c after non-linearity correction is calculated by the following equation for both the three-point bending test piece CB and the short rod test piece SR. ##EQU1## Here, p=(.DELTA.X.sub.0 /.DELTA.X).
Thus, the ISRM core test method requires one test piece for each determination of the fracture toughness, e.g., one test piece for each portion of the underground rock. On the other hand, in order to design underground heat-exchange surfaces, knowledge on the distribution of the fracture toughness over a range of underground depth is necessary. The ISRM core test method takes much time and labor for testing one core for determining the fracture toughness at one portion of rock, and this method is not suitable for determining the values of the fracture toughness of underground rock at different portions thereof.
In short, the conventional ISRM core test method has a shortcoming in that it does not provide any means for continuous measurement of the fracture toughness of underground rock over a range of depth.